In the area of audio loudspeakers it is common to use compression drivers for mid to upper frequencies. There are two main reasons for this. First, the compression driver, when coupled to a waveguide (in this application the terms waveguide and horn are synonymous), provides for much higher electro-acoustical efficiency than a direct radiating loudspeaker can achieve. Second, the waveguide provides for better control over the directional characteristics of the sound radiation than can be achieved with a direct radiator loudspeaker. In order to effectively control the sound radiation, the waveguide must be capable of manipulating the sound wavefronts.
The traditional way of manipulating the wavefront is with diffraction. A small gap is placed within the horn which diffracts the wavefront into a very wide angular coverage spherical wave. This wave is then controlled to a specific angle with by the walls of an additional horn section extending out from the diffraction point.
The traditional approach has two major disadvantages. The first is that the diffraction slot causes a large amount of the wavefront to be reflected back down the horn creating a standing wave and an acoustic resonance, which is highly audible as sound coloration. The second disadvantage is that the new diverging wavefront is not composed of a single wave propagation mode, but contains many propagation modes which are created by the diffraction slot.
These “Higher Order Modes” or HOM are highly undesirable because they cause a loss of coherent wavefront propagation resulting in poor sound quality-mostly in terms of the so-called imaging of the sound system. The HOM propagate by reflecting off of the horn walls as they propagate down the device. This is in contrast with the coherent wavefront propagation which does not reflect off of any internal surfaces as it propagates down the device. It is possible for the coherent wavefront energy to be converted into HOM as the wave front propagates. HOM are also created at any slope or area discontinuities within the waveguide.
Not until Geddes showed the presence of the HOM through his work on waveguides (see Chapter 6 of Audio Transducers, GedLee Publishing, 2002 ISBN 0-9722085-0-X) was the importance of the HOM within the waveguide recognized. Geddes showed that HOM exist in all waveguides and that they play a significant role in wave propagation in a waveguide at the higher frequencies. In order to control the high frequency polar response one has to control the excitation and propagation of the higher order modes. Clearly the wavefront at the exit aperture of the phase plug, which becomes the horns throat input wavefront should be free from the presence of HOM and this has been addressed by Geddes in his recent patent application Ser. No. 10/919,145. This significant point is missing from the entire body of prior art designs for phase plugs.
Other inventors, most notably Tamura and Sato in U.S. Pat. No. 4,893,695 have recognized the importance of a smooth reflection free transmission characteristic from a waveguide. In their patent the inventors disclose the use of an absorbing member as defining “the acoustic path”. Their invention utilizes absorbing material as the actual boundary of the waveguide thereby creating an absorption of the energy bouncing off of the walls. This method does work, but has the disadvantage of requiring a much larger device to accommodate the absorbing boundary. In order to work effectively, the absorbing member must be fairly thick, which causes a large increase in the volume required for the waveguide device.
It is the purpose of this invention to disclose a device which is placed within the boundaries of a waveguide to absorb the HOM as well as any standing waves which may exist within the device that result from reflections from the mouth, etc. The net effect of the use of this device is a substantially improved sound quality for the resulting system.
It has been seen in the marketplace that foam is often placed down the throat of a horn during shipping, presumably to prevent foreign material from entering the device. There is no evidence that this material serves any intended acoustical function and is normally removed prior to usage of the device. There are no teachings that such usage of foam in the throat has any beneficial acoustic effect.